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Interdomain Routing Protocols (BGP)

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Autonomous Systems

• An autonomous system (AS) is a region of the Internet that is administered by a single entity and that has a unified routing policy

• Each autonomous system is assigned an Autonomous System Number (ASN).

• Georgia Tech’s campus network (AS2637) • Rogers Cable Inc. (AS812)• Sprint (AS1239, AS1240, AS 6211, …)

• Interdomain routing is concerned with determining paths between autonomous systems (interdomain routing)

• Routing protocols for interdomain routing are called exterior gateway protocols (EGP)

3

Interdomain and Intradomain Routing

• Routing protocols for intradomain routing are called interior gateway protocols (IGP)– Objective: shortest path

• Routing protocols for interdomain routing are called exterior gateway protocols (EGP)– Objective: satisfy policy of the AS

AS 6 AS 7

AS 4

AS 2 AS 5

AS 1

AS 3

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Interdomain vs Intradomain

• Intradomain routing– Routing is done based on metrics– Routing domain is one autonomous system

• Interdomain routing– Routing is done based on policies– Routing domain is the entire Internet

EGP (e.g., BGP)

AS 2 AS 2

IGP (e.g., OSPF)IGP (e.g., RIP)

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Interdomain Routing

• Interdomain routing is based on connectivity between autonomous systems

• Interdomain routing can ignore many details of router interconnection

AS 1 AS 2

AS 3

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AS Graphs

From: T. Griffin, BGP Tutorial, ICNP 2002

AT&T North America

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Multiple Routing Protocols

• Multiple routing protocols can run on the same router

• Each routing protocol updates the routing table

routingtable

IP Forwarding

routing tablelookup

incoming IP datagrams

outgoing IP datagrams

routing protocol

routing protocol

RIP Process

OSPFProcess

BGP Process

routing table updates

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Autonomous Systems Terminology

• local traffic = traffic with source or destination in AS

• transit traffic = traffic that passes through the AS• Stub AS = has connection to only one AS, only

carry local traffic• Multihomed AS = has connection to >1 AS, but does

not carry transit traffic• Transit AS = has connection to >1 AS and

carries transit traffic

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Stub and Transit Networks

• AS 1, AS 2, and AS 5 are stub networks

• AS 2 is a multi-homed stub network

• AS 3 and AS 4 are transit networks

AS 3

AS 1

AS 4

AS 2

AS 5

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Selective Transit

Example:• Transit AS 3 carries traffic

between AS 1 and AS 4 and between AS 2 and AS 4

• But AS 3 does not carry traffic between AS 1 and AS 2

• The example shows a routing policy.

AS 2AS 1

AS 3

AS 4

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Customer/Provider

• A stub network typically obtains access to the Internet through a transit network.

• Transit network that is a provider may be a customer for another network

• Customer pays provider for service

AS 5

AS 2

Customer/Provider

AS 6

Customer/Provider

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

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Customer/Provider and Peers

• Transit networks can have a peer relationship• Peers provide transit between their respective customers• Peers do not provide transit between peers• Peers normally do not pay each other for service

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

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Shortcuts through peering

• Note that peering reduces upstream traffic• Delays can be reduced through peering• But: Peering may not generate revenue

AS 3

AS 5

AS 2Peers

Customer/Provider

AS 6

Customer/Provider

AS 1Peers

AS 6

Customer/Provider

AS 4

Customer/Provider

AS 6

Customer/Provider

Peers

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Border Gateway Protocol (BGP)

• Border Gateway Protocol is the interdomain routing protocol for the Internet for routing between autonomous systems

• Currently in version 4 (1995)– Network administrators can specify routing policies– BGP is a path vector protocol (However, routing messages

in BGP contain complete routes)

• Uses TCP to transmit routing messages

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Border Gateway Protocol (BGP)

• An autonomous system uses BGP to advertise its network address(es) to other AS’s

• BGP helps an autonomous system with the following:

1. Collect information about reachable networks from neighboring AS’s

2. Disseminate the information about reachable networks to routers inside the AS and to neighboring AS’s

3. Picks routes if there are multiple routes available

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BGP interactions

• Router establishes a TCP connection (TCP port 179)

• Routers exchange BGP routes• Periodically send updates• BGP is executed between two

routers– BGP session– BGP peers or BGP speakers

• Note: Not all autonomous systems need to run BGP. On many stub networks, the route to the provider can be statically configured

AS 1

AS 2

BGP Session

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BGP interactions

AS 2

AS 1

AS 3

• The networks that are advertised are network IP addresses with a prefix, E.g., 128.100.0.0/16

Prefixes reachable from AS 1

Prefixes reachablefrom AS 3

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BGP interactions

• BGP peers advertise reachability of IP networks

• A advertises a path to a network (e.g., 10.0.0.0/8) to B only if it is willing to forward traffic going to that network

• Path-Vector:– A advertises the complete

path to the advertised network– Path is sent as a list of AS’s

this avoids loops

A

B

Advertisepath to 10.0.0.0/24

10.0.0.0/24

BGP Peer

BGP Peer

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BGP Sessions

• External BGP session (eBGP):Peers are in different AS’s

• Internal BGP session (iBGP)Peers are in same AS

• Note that iBGP sessions are going over routes that are set up by an intradomain routing protocol!

AS B

iBGP session

eBGP session

AS A

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iBGP sessions

• All iBGP peers in the same autonomous system are fully meshed

• Peer announces routes received via eBGP to iBGP peers

• But: iBGP peers do not announce routes received via iBGP to other iBGP peers

Update from eBGP session

AS A

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Hot Potato Routing

• Router R3 in autonomous system A receives two advertisements to network X– Which route should it pick?

• Hot Potato Rule: Select the iBGP peer that has the shortest IGP route

• Analogy: Get the packet out of one’s own AS as quickly as possible, i.e., on the shortest path

Route to X

AS A

R1

Route to X

R2

Route to X Route

to X

R3

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Hot Potato Routing

Finding the cheapest IGP route:• Compare the cost of the two

paths– R3 R1 – R3 R2

according to the IGP protocol

• Here: R1 has the shortest path

• Add a routing table entry for destination X

Route to X

AS A

R1

Route to X

R2

Cost=6

R3

Cost=23

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Hot Potato Routing can backfire!

• AS1 would serve its customer (source) better by not picking the shortest route to AS 2

• In fact, customer may have paid for a high-bandwidth service!

AS 2

AS 1

Low bandwidth network

Cost=20

Destination

Source

Cost=5

High bandwidth network

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BGP Message Types

• Open: Establishes a peering session

• Keep Alive: Handshake at regular intervals to

maintain peering session

• Notification: Closes a peering session

• Update: Advertises new routes or withdraws

previously announced routes. Each announced route is

specified as a network prefix with attribute values

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Content of Advertisements

• BGP routers advertise routes

• Each route consists of a network prefix and a list of attributes that specify information about a route

• Mandatory attributes:

ORIGIN

AS_PATH

NEXT_HOP

• Many other attributes

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ORIGIN attribute

• Originating domain sends a route with ORIGIN attribute• ORIGIN attributes also specifies if the origin is internal to the AS or not

AS 1

AS 2 AS 4

AS 5

AS 3

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

10.0.1.0/8, ORIGIN {1}

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AS-PATH attributes

• Each AS that propagates a route prepends its own AS number– AS-PATH collects a path to reach the network prefix

• Path information prevents routing loops from occurring• Path information also provides information on the length of a path (By

default, a shorter route is preferred)• Note: BGP aggregates routes according to CIDR rules

AS 1

AS 2 AS 4

AS 5

AS 3

10.0.1.0/8, AS-PATH {2,1}

10.0.1.0/8, AS-PATH {3,1}

10.0.1.0/8, AS-PATH {4,2,1}

10.0.1.0/8, AS-PATH {1}

10.0.1.0/8, AS-PATH {1}

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NEXT-HOP attributes

• Each router that sends a route advertisement it includes its own IP address in a NEXT-HOP attribute

• The attribute provides information for the routing table of the receiving router.

AS 5 AS 1

AS 3

128.100.11.1 128.143.71.21

10.0.1.0/8, NEXT-HOP {128.100.11.1}

10.0.1.0/8, NEXT-HOP {128.143.71.21}

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Connecting NEXT-HOP with IGP information

AS 1

AS 3

128.100.11.1/24 192.0.1.2

eBGP

iBGP

R1IGP router

10.1.1.0/8, NEXT-HOP {128.100.11.1}

10.1.1.0/8, NEXT-HOP {128.100.11.1}

Dest. Next hop128.100.11.0/24 192.0.1.2

At R1:

Dest. Next hop10.1.1.0/8 128.100.11.1

Routing table

BGP info

Dest. Next hop128.100.11.0/24 192.0.1.2

10.1.1.0/8 192.0.1.2

Routing table

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Route Selection

• Router may get more than one route to an address• Rules for selecting a route (in order of priorities):

– Preferences can be advertised as an attribute– Shorter routes are preferred– Close next-hop is preferred

• Router may not want to advertise some routes

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Importing and Exporting Routes

• An AS may not accept all routes that are advertised

• An AS may not advertise certain routes

• Route policies determines which routes are filtered

• If an AS wants to have less inbound traffic it should adapt its export rules

• If an AS wants to control its outbound traffic, it adapts its import rules

Control Inbound

traffic

AS A

Change export rules

Change import rules

Control Outbound

traffic

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Routing Policies

• Since AS 5 is a stub network it should not advertise routes to networks other than networks in AS 5

• When AS 3 learns about the path {AS1, AS4}, it should not advertise the route {AS3, AS1, AS4} to AS 2.

AS 1

AS 3

AS 5

AS 2

Peers

Customer/Provider

Peers

Customer/Provider

AS 4

AS 6

Peers

Customer/Provider

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Traffic Often Follows ASPATH

• In many cases, packets are routed according to the AS-PATH

• However, in some cases this is not true

(Here: AS 2 filters routes with a long prefix)

AS 2 AS 5AS 3AS 1128.100.0.0/16

128.100.0.0/16, AS-PATH {3,2,1}

AS 2 AS 5AS 3AS 1128.100.0.0/16

128.100.0.0/16, AS-PATH {3,2,1}

AS 4

128.100.22.0/24, AS-PATH {4}

128.100.22.0/24

128.100.0.0/16, AS-PATH {1}

128.100.0.0/16, AS-PATH {2, 1}

Does not advertise /24 networks

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Short AS-PATH does not mean that route is short

• From AS 6’s perspective – Path {AS2, AS1} is short– Path {AS5, AS4, AS3, AS1} is

long

• But the number of traversed routers is larger when using the shorter AS-PATH

AS 1

AS 3

AS 4

AS 5

AS 2

AS 6

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BGP Table Growth

Source: Geoff Huston. http://www.telstra.net/ops/bgptable.html on August 8, 2001

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BGP Issues

• BGP is a simple protocol but it is very difficult to configure

• BGP has severe stability issue due to policies BGP is known to not converge

• As of July 2005, 39,000 AS numbers (of available 64,510) are consumed